1. Field of the Invention
This invention relates to fuel cell systems, and more particularly provides method and apparatus for cooling and recirculating fuel cell discharge products.
2. Description of the Prior Art
In a fuel cell system, typically including a plurality of individual cells arranged in a stack, process reactants including a fuel, such as hydrogen, and an oxidant, such as air, undergo an electrochemical reaction, through an electrolyte such as phosphoric acid, to produce an electric current. The reaction is exothermic, thus also providing heat, and reaction products in a hydrogen and oxygen process include high temperature water vapor. The water is formed predominantly on the cathode side of the reacting cells. The heat of the electrochemical reaction must be continuously removed in order to prevent the cell temperature from rising to a point which could damage cell components and system efficiency.
Heat and water have typically been removed from the fuel cell stack by circulating excess air across the cathode. The excess air becomes humidified and heated as it passes through the cell. The excess air has typically been cooled by passage through heat exchangers, and dehumidified in separate dehumidification equipment. The heat exchanger and dehumidification functions have been performed separately, and in distinct units. A high temperature prime mover, such as a pump or blower, has also been needed in order to circulate the excess air through a cooling loop. Particularly for high system pressures, the size, complexity and cost of the prime mover is a substantial factor in overall system efficiency, particularly where the prime mover is exposed directly to high temperature reaction products.
More recently proposed have been cooling loops which circulate a liquid or a gaseous coolant through selected portions of the fuel cell stack. Such systems, however, additionally require a substantial circulation pumping device and elaborate leak tight manifolding systems.
It is desirable to provide fuel cell systems which effectively remove heat from the fuel cell stack with simplified components and without substantial detraction from system efficiency.